The invention relates generally to a method for iron-making, and more particularly to a FHM furnace method for iron making, using pre-reduced iron, where said FHM furnace method has improved economics, quality and environmental aspects.
The present invention relates to means for utilizing a hot pre-reduced iron ore charge to the hearth furnace for the purposes of increasing productivity, improving overall process thermal efficiency, reducing sulfur and lowering emission loading. The invention in turn relates to charging hot, highly metallized DRI product to an ITmk3 xe2x80x9cFinisherxe2x80x9d furnace to effect melting of the DRI to produce nuggets of pure iron (steel), which contain no gangue components. Resulting steel nuggets can then be charged to an electric furnace for further processing (i.e., melting, alloying) into molten steel, which is suitable for casting into billets, and other desirable steel products. Hot, highly metallized product can preferably be sourced from a Midrex DRI shaft furnace or a rotary hearth furnace.
ITmk3 furnace technology was developed by Kobe Steel, LTD of Osaka, Japan, to separate metal from iron ore using coal. Briefly, ITmk3 technology employs a pellet of finely ground iron ore compounded with coal dust and a binder, to metallize the iron oxide into iron, melt and express slag, and then a means to separate a hot iron nugget from the slag.
Liquid (molten) steel is primarily produced by two methods; blast furnace/basic oxygen furnace (BF/BOF), or the electric arc furnace (EAF). The BF/BOF route depends on the production of hot metal from the blast furnace (typical liquid pig iron analysis of greater than 4% carbon, 0.5xcx9c1.0% silicon, 0.05% sulfur and 0.04% phosphorus) as a source of both virgin iron units and xe2x80x98fuelxe2x80x99 to the BOF converter. The charge to a BOF converter typically contains 20xcx9c30% scrap steel and the remainder liquid hot metal. On the other hand, the EAF can theoretically process a charge of essentially 100% scrap steel. However, the preferred economical charge for an EAF may contain 5 to 15% solid pig iron and/or other alternative iron sources, e.g., 10xcx9c85% or more DRI (direct reduced iron) and/or HBI (hot briquetted iron). The solid pig iron, similar to the liquid pig iron, acts as both a source of virgin iron units and carbon/silicon fuel which, when processed correctly, reduces the electrical energy requirement for melting the scrap steel charge. Therefore, the use of oxygen is common to both BF/BOF and EAF steel-making routes for the purpose of promoting chemical energy from oxidation reactions (heat generated by oxidation of carbon, silicon and iron). Also, slag conditioners (primarily burnt lime or dolomitic lime) are added to the charges of the BOF or EAF for the purpose of desulfurization of the liquid steel. The resultant slag volume can typically be 100xcx9c200 kg/mt of liquid steel, with 30xcx9c40% by weight of the slag being iron oxide.
The problem presented is to devise an economical, low capital cost iron-making or steel-making process that exhibits both a high iron yield and high degree of energy efficiency for the production of either an iron shot intermediate product, or a liquid (molten) steel product that is suitable for further processing into semi-finished steel products (slabs, blooms, billets, sheet). The present invention is a process that achieves production of iron shot, or direct steel-making by means of coupling an established technology for iron ore pre-reduction, such as the Midrex natural gas-based direct reduction technology, with a modified moving hearth furnace, where the modified moving hearth furnace is a novel coal-based moving xe2x80x9cfinisherxe2x80x9d hearth melter (FHM) furnace with a refractory surface, whereby there is no necessity for decarburization of the iron product exiting the FHM furnace.
Applicant is aware of the following related U.S. Pat. Nos.:
Hoffman, U.S. Pat. No. 4,731,112, discloses a method of making a molten ferroalloy product in a melting furnace by charging a briquette consisting essentially of metallized iron, granulated alloying metal oxide, into a carbon source, such as coke breeze, to the melting furnace, burning solid carbonaceous material to reduce the alloying metal oxide to metallized form and to heat the charge to form a molten ferroalloy product. Fluxes and slag formers are also charged to the furnace as required.
Sawa et al, U.S. Pat. No. 6,126,718, discloses a method of producing iron from a metal-containing reducible material comprised of iron oxide compounded with metal reducing materials in a rotary hearth furnace. In the Sawa ""718 method, the reducible material is filled into horizontal trays, where said horizontal trays resemble ice trays. The filled horizontal trays are conveyed through a hearth furnace that is fired with a hot reducing, mixture of gases. The reducible material, which contains iron oxide, is converted into iron. In the latter zones of the rotating hearth furnace, the reducible mixture melts, forming liquid iron having a cap of slag. The iron and slag are cooled, and then separated by screening into iron and slag.
The present invention performs iron ore pre-reduction by means of established gas or coal-based direct reduction technologies which can use low sulfur syngas, natural gas or coal, and which have:
A high degree of process control,
High process fuel and thermal efficiency,
Low gangue in the reduced iron product (typically less than 5%) translates directly into reduced slag generation in the FHM furnace,
Production of highly reduced iron of uniform chemistry, carbon content of 0-6.7%, and low sulfur content by virtue of using natural gas, syngas, etc., as the reductant source, where natural gas and the like have very low sulfur content, especially as compared to certain grades of coal.
High quality reduced iron product is charged (preferably hot) to a moving xe2x80x9cfinisherxe2x80x9d hearth melter (FHM) furnace whereby controlled melting can be effected to produce iron shot nuggets (having a content of 0.01-4% C) that are free of all gangue material. Any carbon contained in the hot reduced iron product is able to reduce a portion of, or nearly all of, the residual iron oxide.
Prior to charging the hot reduced iron to the FHM furnace refractory surface, the hearth surface is covered with hearth conditioning materials, comprised of a hearth conditioner which is a hearth carbonaceous material such as graphite, anthracite coal, petroleum coke, etc., and may also contain refractory compounds such as SiO2, CaO, alumina, bauxite, CaF2 (fluorspar), magnesia, magnesite, etc. A portion of the hearth carbonaceous material acts as the source of solid carbon that diffuses into the metallic iron to lower the effective melting point and to promote formation of nuggets. The remainder of the hearth conditioning material acts as a protective layer which supports the molten iron nugget and the nascent separated slag, and prevents penetration of liquid iron/slag into the hearth refractory. In addition, some of the carbonaceous material is oxidized by the burner combustion products to form carbon monoxide. Carbon monoxide is a reductant, and it acts to reduce iron oxide to elemental iron. Provision is also made in the invented process to coat or dust the outer surface of the pre-reduced metallized iron with a powdered carbonaceous material, where the powdered carbonaceous material is usually very similar to the hearth conditioner. The pre-reduced metallized iron is coated just prior to being charged onto the hearth surface. The pre-reduced metallized iron is charged hot (500xcx9c900xc2x0 C.), and the energy required for subsequent heating/melting is significantly lower than that required for conventional RHF (rotary hearth furnace) operation which includes the energy required for initial heating and pre-reduction. It is estimated that the burner fuel requirement is  less than 0.7 Gcal/mt-nuggets. Also, the FHM furnace residence time is significantly reduced, xcx9c50%, from 12 to approximately 6 minutes or less. Cold, reduced iron can also be charged to the FHM furnace, but the energy requirement is greater, and the residence time is longer. The nominal discharge temperature of the steel nuggets from the FHM furnace is in the range of 1,100xcx9c1,300xc2x0 C. If the carbon content of the nugget is very low ( less than 0.3%), then the discharge temperature is higher, in the range of 1300 to  greater than 1430xc2x0 C. If higher carbon content is desired, then lower discharge temperatures are required. The FHM furnace is capable of facilely producing steel nuggets that have a higher carbon content than the usual RHF DRI product. Therefore, when the FHM furnace nugget carbon content exceeds 0.3%; the resultant nugget discharge temperature from the FHM furnace must be reduced.
The atmosphere in the FHM furnace (reducing in nature such that a minimum of 10% combustibles is present) can be generated by air/fuel burners, where fuel is preferably (but not limited to) fuel gas or natural gas (or an equivalent mixture of fuel gases having a heating value similar to natural gas), and process air, where both the fuel gas and the process air are preheated to 450xcx9c700xc2x0 C. by means of conventional heat recuperation means applied to the FHM furnace off-gas. Other suitable fuels could be waste oil, coal, etc. Some possibilities for generating a portion or possibly all of the fuel gas requirements for the FHM furnace burners can be achieved by either heating (de-volatizing) or calcining the coal required for the hearth conditioning step by diverting a portion of the conditioned spent flue gas exiting the FHM furnace to an indirect heater, or a fluid bed type calciner.
After being discharged from the FHM furnace, the hot nuggets (at a temperature of 900xcx9c1,450xc2x0 C.) are hot screened, preferably by a water cooled moving screen system whereby the product nuggets are physically separated from the hearth conditioning material and small amounts of slag. The iron/steel nuggets can be quenched, or they may be fed directly to a final melter (preferably an electric furnace, but more preferably a channel induction melting furnace having a substantial liquid steel heel), while the underside carbonaceous hearth protection material and any mini-nuggets can be cooled, magnetically separated and then recycled back to the FHM furnace. The channel induction furnace is the preferred melter for this application due to the fact that the energy requirement for melting the product steel nuggets is low (120xcx9c200 kWh/mt), the melter charge is virtually 100% metallic steel nuggets with no gangue that can attack the refractory lining in the heating coils, the melter atmosphere can be easily controlled against reoxidation, and the expected melter maintenance is low. The nuggets are then re-melted in the final melter where the liquid metal bath chemistry can be adjusted by small alloying additions. The final melter can then be tapped (intermittently or continuously), and the molten metal stream directed to a tundish that meters liquid steel to a continuous caster. The invented process and apparatus eliminates the need for ladles and large overhead cranes.